After Microsoft's Gabe Aul teased the world via Twitter this morning, Windows 10 Mobile Preview Build 10586 released to fast-ring Insiders Wednesday afternoon. It comes with a slew of improvements—and one big caveat for Insiders who already have Build 10581 on their phones.

Or rather, a big bug: "After we released Build 10581 to Windows Insiders in the Fast ring," Aul explained in his blog post, "we discovered a bug in the build that will cause the filesystem to become partially corrupted after doing a factory reset." According to Aul, this bug probably didn't affect anyone while using Build 10581, but it could cause problems when upgrading to Build 10586. Specifically, your phone could go into an infinite reboot cycle.

There is a fix: A hard reset should take you safely to Build 10586. In boldface, Aul urges users to do one important thing: back up their phones before they upgrade.

The physical camera button is back—it had stopped working for some users.

Why this matters: This isn't the first buggy Windows 10 Mobile build. Fast ring Insiders may be a hardy lot, but this big bug merits some extra care. Now that you're cautiously upgrading to Build 10586, let us know whether it was worth the hassle in the comments.

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